George Dvorsky

A new study published in Current Biology is giving new meaning to "caught in the act." Researchers have observed that bats can use their sonar to home in on flies who are in the midst of copulation. Turns out noisy sex can have worse consequences than just embarrassment.

Scientists have suspected that copulation can put animals at risk for predators, but they've lacked the empirical evidence to support this claim, until now.

Scientists also know that bats like to eat flies, but they couldn't understand how a nocturnal animal is able to prey upon a diurnal one. Flies, like humans, sleep at night, thus making them completely invisible to the bats who rely on echolocation for hunting.

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But flies, like humans, also appear to do other things at night, beside sleep.

The study, which was conducted by Stefan Greif of the Max Planck Institute for Ornithology, took a look at Natterer's bats living in cowsheds near Marburg, Germany. He noticed that flies emit a burst of broadband when they're mating, an acoustic signature that's likely emanating from the male's wing-fluttering. The bats have learned to identify these special click-like signals, thus allowing them to swoop in on the unsuspecting couple and earn a super-sized meal.

To conduct their work, the researchers set up cameras in the cowshed to catch a glimpse of the action. Their videos show that flies are extremely reluctant to fly at night (for good reason), and that bats almost never attack flies who walk on the ceiling.

But once the flies start having sex, their risk of being detected and eaten by the bats rises dramatically.

Greif's team spent the better part of four years making these videos, observing the actions of over 9,000 flies and chronicling over 1,100 acts of them doing the nasty (wow, talk about a lengthy and detailed study). What they discovered was that flies who are mating get eaten 5% of the time. And in about 60% of those cases, the bats were able to gobble-up both of the flies.

In order to ensure that it was the sounds that were alerting the bats to the flies' presence, the researchers set up mounted, dead fly pairs on the shed ceiling in a position they usually take when mating. Not surprisingly, the bats failed to notice the pornographic decoys.

The researchers suspect that other predators may employ the same strategy — a suspicion that stems from the theory that mating, as important as it is to the survival of a species, often puts organisms in terrible peril. And as Greif notes in the study, not only are animals noisy and conspicuous during copulation, they're also distracted — an often deadly combination when predators are around.

And, in fact, as other research suggests, mating can be deadly. As Ed Yong points out in Discover, water sliders, amphipods, and Australian plague locusts face similar risks when mating. The locusts in particular have it pretty rough. Mating locusts are targets for parasitic wasps, who sting, paralyze and bury the female, while the male, who is unable to detach himself, is dragged along by his genitals and also entombed.

Ouch. The flies in Greif's study would seem to have it considerably easier.